The abatement of emissions, particularly the oxides of nitrogen (NOx) is gaining ever increasing attention and significant resources are being applied to the associated problems. In the burning of a fuel, the oxides of nitrogen result from the high temperature oxidation of the nitrogen from the air and the oxidation of nitrogen in the fuel. In liquid fuels, the fuel nitrogen may be present in the form of any of the nitrogen bearing hydrocarbon compounds of which pyridine is one example. In gaseous fuel, the fuel nitrogen may be present in the form of ammonia or some other nitrogen compound.
The level of NOx emissions is being regulated by many governmental agencies. For example, the United States Environmental Protection Agency has proposed rules for stationary gas turbines which limit the emission of NOx to 75 ppmv corrected to 15% oxygen by volume and a heat rate of 13646 BTU/Kw.Hr. 42 Fed. Reg. page 53782 (Oct. 3, 1977). Some governmental codes are now in effect as exemplified by Los Angeles County Rule No. 67 issued by the Los Angeles County Air Pollution Control District which requires the control of NOx emission from industrial gas turbines in Southern California.
Presently, the NOx code requirements and proposed requirements can only be met by injecting steam or water directly into the reaction zone of the stationary gas turbine combustor. However, as fuel costs continue to increase, a greater emphasis is being placed on the heat rate of the power generating system and while water injection is very effective at reducing NOx and will allow an increase in power output, there is a small but significant increase in heat rate. In addition, an advantage of a gas turbine over other types of power generating plants has been its ability to operate in a wide variety of climates without the need for a cooling water supply. Still further, difficulties are encountered in supplying water of sufficient purity to prevent damage to the turbine. Multistage combustion systems are now under development but these systems are relatively new, complex and expensive.
Systems have been known in which NOx is minimized in a gas turbine engine and in other systems in which an exhaust gas from a preliminary combustor is utilized as the primary source of air for the main combustor. See, e.g., U.S. Pat. Nos. 3,792,581, 4,009,689 and 4,147,141. Rice, 3,703,807, teaches a combined gas-steam turbine power plant in which a portion of the exhaust gas from the boiler stack is recycled to the compressor. Stettler, U.S. Pat. No. 3,969,892 discloses a gas turbine system in which a portion of the exhaust gas from the burner is recycled through a heat exchanger and then back into the burner with a resulting reduction in nitrogen oxide in the exhaust; to effectively reduce the NOx emissions, the rate of the recycled gases is about twice the weight of the entering fresh air. Lockwood, U.S. Pat. No. 3,949,548, teaches an exhaust gas recirculation system in which a portion of the exhaust gas is cooled and recirculated through a compressor with a resulting reduction in nitrogen oxide. This system requires two separate compressors, one for ambient air and one for the recycled exhaust gas. The use of exhaust gas recirculation has not been applied to stationary gas turbines in practice because of the problems associated with cooling the exhaust gases, the requirements for high fluid flow as in Stettler, or the complications and expense of multicompressor systems as in Lockwood.
It is the object of this invention to provide a means for controlling NOx emissions from a conventional stationary gas turbine combustion system by means of exhaust gas recirculation while overcoming the prior art problems associated with cooling the exhaust, mass flow and complex flow routes and expensive equipment.